Many studies have conventionally been conducted on light-sensitive lithographic printing plate precursors using a photo-polymerization system. Specifically, development of light-sensitive lithographic printing plate precursors having a high printing durability utilizing toughness of a layer of the photo-polymerization system and development of laser printing plate precursors utilizing a highly sensitive photo-polymerization-initiating system which is sensitive to an argon ion laser (488 nm), FD-YAG laser (532 nm) violet laser (405 nm) or the like have been made.
Many of the light-sensitive lithographic printing plate precursors comprise a support of grained and anodized aluminum plate having provided thereon a photo-polymerizable composition layer comprising an addition-poloymerizable ethylenic double bond-containing compound, a photo-polymerization initiator, an organic polymer binder and a thermal polymerization inhibitor and a barrier layer for oxygen that inhibits polymerization on the photo-polymerizable composition layer.
The light-sensitive lithographic printing plate precursor is imagewise exposed to a desired image to polymerize and cure the exposed area, followed by removing the unexposed area with an alkali aqueous solution (development), thereby obtaining a relief image. In the unexposed area, the aluminum support is revealed and, since the surface of the aluminum support is hydrophilic, it retains water and repels an oily ink, whereas the image area (area which have been cured by the imagewise exposure) is oleophilic and repels water and receives an ink, thereby exhibiting function as a printing plate.
The processes of exposure and development have conventionally been manually conducted using a contact exposing machine and an automatic developing machine. With the spread of CTP (Computer-To-Plate) system, however, pre-press steps including film working are omitted, and digital data processing permits a series of processes. Thus, it has become common to mechanically and continuously conduct the exposure process and the development process by loading unexposed printing plate precursors together on a drawing machine (plate setter). Therefore, cases increase wherein the printing plate precursor is scratched in the plate setter during conveyance of the printing plate precursor to an exposing section, mounting and removal of the printing plate precursor on and from the exposing section and conveyance to an automatic developing machine, which has become a serious problem. The formation of scratch on the light-sensitive layer side results in crack of the overcoat layer, thus causing serious problems that spoil the essential functions as a printing plate precursor, for example, polymerization failure or dark polymerization of the unexposed area due to pressure effects. Accordingly, countermeasures have been taken from the apparatus side, for example, development of a conveying method which enables the surface of the light-sensitive material to avoid contact with the apparatus, precise polishing of rollers and the like. On the other hand, regarding the printing plate precursor, measures for increasing adhesion force between the overcoat layer and the light-sensitive layer and measures for adjusting film properties and slipping properties of the overcoat layer have been investigated. Consequently, a practically satisfactory level can be achieved. On the contrary, scratch on the aluminum surface (back surface) opposite to the light-sensitive layer side has been comparatively no concern because the scratch exerts no detrimental influence on printing so long as it is not so serious in case of a small number of prints. Recently, however, with popularization of CTP and increase in number of prints, the scratch on the back surface causes problems. When aluminum waste moves to the light sensitive layer surface side, problems arise that the aluminum waste causes scratch on the surface or that since the aluminum waste acts as a light shielding agent, the area which should essentially be exposed is not exposed. Therefore, development of a printing plate precursor that scarcely suffers formation of scratch on the aluminum back surface has been demanded.
No known documents have been found on the countermeasures against the formation of scratch on aluminum back surface but, though for different purposes, several techniques for improving the aluminum back surface have been reported. For instance, as a countermeasure to prevent formation of scum in an alkali developing solution by preventing dissolution of aluminum, which causes the scum, from the aluminum support into the alkali developing solution, there have been disclosed a method of providing a coating layer comprising an organic polymer compound on the aluminum back surface opposite to the light-sensitive layer side (see, e.g., Patent Document 1), a method of providing a metal compound obtained by hydrolysis and polycondensation of an organic metal compound or an inorganic metal compound on the back surface of a light-sensitive lithographic printing plate precursor (see, e.g., Patent Document 2), and a method of providing a layer comprising a sol-gel reaction solution obtained by hydrolysis and polycondensation of an organic metal compound or inorganic metal compound, a polymer compound and a plasticizer on the back surface of an aluminum substrate, which has been anodized on both surfaces thereof, opposite to the photo-polymerizable layer side (see, e.g., Patent Document 3). However, these methods are insufficient as countermeasures against formation of scratch, and more improvement has been desired. The presence of an anodized film on the back surface of the aluminum plate has been known (see, e.g., Patent Document 3). However, the anodized film is formed by going round of electric field to the back side at the formation of the anodized film on the light-sensitive layer side. Accordingly, the thickness of the anodized film on both edges of an aluminum coil is larger and the thickness thereof in the central portion is smaller so that the anodized film is uneven in thickness and insufficient as countermeasures against the formation of scratch utilizing hardness of the anodized film.
Also, a method of providing an organic polymer having a high Tg on the back surface for the purpose of preventing adhesion to the light-sensitive layer surface has been disclosed (see, e.g., Patent Document 4). However, this method is also insufficient as countermeasures against the formation of scratch. Therefore, improvements thereof have been desired.    Patent Document 1: EP-A-0490515    Patent Document 2: JP-A-6-35174    Patent Document 3: JP-A-8-240914    Patent Document 4: EP-A-1031881